1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a backlight unit and LCD device using the same. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for improving uniformity and color mixture of the backlight.
2. Discussion of the Related Art
Generally, a cathode ray tube (CRT) is mainly used as one of various display devices, such as a monitor of a television (TV), measuring instrument, information terminal or the like. However, the CRT cannot meet the demand for an electronic product having a reduced size and weight. For this reason, many flat display devices, such as an LCD using electric-field optical effect, a plasma display panel (PDP) using gas discharge, and an electroluminescent display (ELD) using electroluminescence, were developed to substitute in place of the CRT. Among those flat display devices, the LCD has attracted considerable attention because of its small size, light weight, and low power consumption. Hence, there has been a great demand for the LCD to be utilized as a laptop computer monitor, a desktop computer monitor, and a large-scale information display.
The LCD device is a light-receiving device that displays images by adjusting a quantity of a light source. The light source is a separate entity that may be a backlight for applying light to an LCD panel. The backlight may be either an edge type or a direct type depending on an installation location of a lamp unit of the backlight. Currently, there are various types of light sources such as EL (electroluminescence), LED (light emitting diode), CCFL (cold cathode fluorescent lamp), and HCFL (hot cathode fluorescent lamp) that can be used as the light source. In particular, the CCFL has been widely utilized for a large-scale color TFT-LCD because of its thin profile, long endurance and low power consumption.
The CCFL includes a fluorescent discharge tube in which Hg gas with Ar, Ne and the like is sealed to utilize a penning effect. Electrodes are provided at both ends of the tube. A negative electrode is formed flat to have a wide area. In case of voltage impression, electric charged particles within the discharge tube collide with the flat negative electrode to generate secondary electrons like sputtering. The generated secondary electrons excite neighbor elements to generate plasma. The excited elements discharge strong UV-rays that excite a fluorescent substance to radiate a visible ray.
In the edge type backlight, a lamp unit is provided to a lateral side of a light-guiding panel. The lamp unit includes a lamp emitting light, a lamp holder mounted in both ends of the lamp to protect the lamp, and a lamp reflector having one side fitted in the lateral side of the light-guiding panel to reflect the light emitted from the lamp toward the light-guiding panel. Such an edge type backlight is applied to a small-scale LCD device as a monitor of a laptop or desktop computer. The LCD device utilizing the edge type backlight is advantageous in light uniformity, long endurance and slim size.
In the direct type backlight, a plurality of lamps are arranged in a row to directly illuminate a front side of an LCD panel. The direct type backlight has a higher light efficiency than that of the edge type backlight and is mainly applied to a large-scale LCD (over 20 inches) requiring high luminance. An LCD with the direct type backlight is utilized as a large-scale monitor or television to be driven longer than a laptop computer, and includes more lamps. Hence, it is possible that some of the lamps in the direct type backlight are not lighted due to the failure or long-term use endurance of the lamps. As a result, a part failing in lighting the corresponding lamp becomes remarkably darker than the normal part to be directly recognized on the screen. Accordingly, the direct type backlight requires frequent replacements for lamps and needs to have a configuration facilitating assembly/disassembly of the backlight unit.
As noted above, the LCD adjusts the light quantity transmitted via the screen using liquid crystals and determines the color and brightness using the adjusted light. Hence, the LCD device is different from those of general display devices in the following aspects. For instance, the LCD device provides a viewing angle with considerably varying image quality according to an angle for viewing a screen, a transmittance according to a projective type light emitting display, a color reproducibility of how much a transmitted light passing through a color filter can reproduce R, G and B colors, a luminance indicating brightness of a picture, and an after image resulting from an image of long-term retention.
The LCD expands its field to a desktop PC monitor and a home TV from a display of a portable product. Despite its physical advantages of lightweight, flatness, smallness and shortness, the LCD is less advantageous than the CRT in color reproducibility, luminance and the like. A conventional notebook monitor LCD has color reproducibility 40˜50% lower than a color TV according to a system adopted by NTSC (National Television System Committee). However, it can meet the user's demand with its color reproducibility only. Thus, the LCD market demands the development of an LCD that can implement color reproducibility exceeding that of the CRT for use in a TV.
A general multi-color LCD, which includes a liquid crystal panel, a backlight and a color filter, implements various colors by separating a white light projected from the backlight having a 3-wavelength fluorescent lamp into three colors of R, G and B and by adding and mixing the three colors. When blue, green and red LEDs are simultaneously used in implementing a white color, many problems take place in its application. In particular, its application is substantially difficult due to the technical difficulty of producing the white color by collecting different colors coming from locations of the respective LEDs. Hence, to implement the white color, one LED can emit three wavelengths having predetermined intensity at least. Thus, there has been a strong desire to develop a backlight that can provide the advantages of a small SMD (surface mounting device) LED of a mobile phone having very low power consumption as well as the advantages of the CCFL of a notebook computer having excellent color reproducibility.
A backlight according to the related art is explained with reference to FIG. 1. FIG. 1 is a cross-sectional view illustrating a backlight unit according to the related art. As shown in FIG. 1, the backlight unit is provided with a fluorescent lamp 1, a light-guiding panel 2, a diffusion substance 3, a reflector sheet 4, a diffuser sheet 5 and a prism sheet 6. When a voltage is applied to the fluorescent lamp 1, remaining electrons existing in the fluorescent lamp 1 migrate into a positive electrode. The migrating electrons collide with Ar. The Ar is excited to increase positive ions. The increased positive ions then collide with a negative electrode to generate secondary electrons. When the generated secondary electrons flow within a tube to initiate electric discharge, a flow of electrons by the electric discharge collide to ionize with Hg vapor to emit UV and visible rays. The emitted UV rays excite a fluorescent substance coated on an inner wall of the fluorescent lamp 1 to emit light of visible ray.
The light-guiding panel 2 is a wave-guide that projects a surface light source by introducing the light emitted from the fluorescent lamp 1 inside. The light-guiding panel 2 is formed of PMMA (polymethyl methacrylate) resin having good light transmittance. Factors relating to incident light efficiency of the light-guiding panel 2 include light-guiding panel thickness to lamp diameter, distance between the light-guiding panel 2 and the lamp 1, and a lamp reflector shape. The incident light efficiency is raised by laying the fluorescent lamp 1 aslant in a thickness direction deviating from a center of the light-guiding panel 2. The light-guiding panel 2 of the LCD backlight may be one of a print type light-guiding panel, a V-cut type light-guiding panel, and a dispersion light-guiding panel.
The diffusion substance 3 includes SiO2 particles, PMMA, solvent and the like. The SiO2 particles are used for light diffusion and have a porous structure. The PMMA is used in attaching the SiO2 particles to a lower surface of the light-guiding panel 2. The diffusion substance 3 is coated on a lower surface of the light-guiding panel 2 in a dot form. A dot area is increased step by step to obtain a uniform surface light source on an upper part of the light-guiding panel 2. That is, an area rate per unit area occupied by the dot close to the fluorescent lamp 1 is small, whereas an area rate per unit area occupied by the dot distant from the fluorescent lamp 1 is large. Moreover, the dot may have various shapes. If the area rate per unit area is equal, the effect of the same brightness can be obtained from the upper part of the light-guiding panel 2.
The reflector sheet 4 is provided to a rear end of the light-guiding panel 2 to enable the light projected from the fluorescent lamp 1 to enter an inside of the light-guiding panel 2. The diffuser sheet 5 is provided over the light-guiding panel 2 and has a dot pattern coated thereon to provide uniform luminance according to a viewing angle. The diffuser sheet 5 is formed of PET or PC (polycarbonate) resin. A particle coating layer is provided on an upper part of the diffuser sheet 5 to play a role in diffusion. The prism sheet 6 serves to enhance front luminance of light that is transmitted through the upper part of the diffuser sheet 5 to be reflected. The prism sheet 6 enables a specific-angle light to transmit. The rest-angle incident light returns to a lower part of the prism sheet 6 due to total internal reflection. The returning light is reflected by the reflector sheet 4 attached to the lower part of the light-guiding panel 2.
The backlight unit of FIG. 1 is fixed to a mold frame. A display unit provided to an upper surface of the backlight unit is protected by a top chassis. The top chassis and the mold frame are assembled together to accommodate the backlight unit and display units in-between.
Other backlight units according to the related art are explained with reference to drawings. FIG. 2 is a perspective view illustrating an optical wave guide tube plate of a backlight unit according to the related art. FIG. 3 is a layout view illustrating another backlight unit according to the related art. FIG. 4A and FIG. 4B are cross-sectional views illustrating the backlight unit of FIG. 3 taken along lines I-I′ and II-II′, respectively.
As shown in FIG. 2, a rectangular optical wave guide tube plate 10 of the related art backlight unit is formed of a transparent substance. The optical wave guide tube plate 10 includes a light discharging surface 11 at a couple-out upper lateral side, a lower lateral side 12 provide to an opposite side to the light discharging surface 11 and four lateral sides 13 to 16. A plurality of cavities 20 for a light source are indented to the lower lateral side 12 of the optical wave guide tube plate 10 to extend in a direction of the light discharge surface 11. R/G/B LED lamps (not shown) are provided under the plurality of cavities 20, respectively. Preferably, the plurality of cavities 20 are evenly distributed on the optical wave guide tube plate 10 in a regular grid arrangement.
The thus-configured backlight unit using the R/G/B LED lamps is advantageous in improving color reproducibility not using Hg. To mix and discharge colors coming from the R/G/B LED lamps, each of the cavities needs to be provided with an upper lateral side and a lateral sidewall to correspond to the light discharge surface 11. The upper lateral side should be coated with a first reflecting layer and a lower lateral side of each of the cavities should be coated with a second reflecting layer. An edge of each of the cavities 20 should be enclosed by a third reflecting layer. Moreover, it needs to be complicatedly configured to bring about coupling of the LED lamp light between the optical light guide tube plate 10 and each of the cavities 20 through the lateral sidewall.
The backlight unit of FIG. 3 is provided with a direct type intermediate light-guiding panel. As shown in FIG. 4A and FIG. 4B, the related art backlight includes a plurality of lamp array units 30 (see FIG. 3), each having a plurality of LED lamps 31 arranged in one direction, a light dispersion member 34 arranged over the lamp array units 30 for diffusing light to transfer to an LCD panel, an intermediate light-guiding panel 32 provided between the LED lamps 31 and the light dispersion member 34, a plurality of reflector sheets 33 provided beneath the intermediate light-guiding panel 32 to oppose the LED lamps 31, respectively, and an outer case 40 having the LED lamps 31 fixed thereto to support the intermediate light-guiding panel 32 and the light dispersion member 34.
In the related art backlight unit, the transparent intermediate light-guiding panel 32, which has the reflector sheets 33 attached to its bottom to confront the LED lamps 31, respectively, is provided in the vicinity of the LED lamps 31, so that R/G/B colors emitted from the LED lamps 31 can be well mixed. Moreover, by the reflector sheets 33 provided beneath the intermediate light-guiding panel 32 to confront the LED lamps 31, the strong lights emitted from the LED lamps 31 are prevented from directly coming to the LCD panel, respectively.
However, in the related art LCD backlight unit, the intermediate light-guiding panel 32 may be hung down in a direction of the LED lamps 31 by heat, vibration, shock or the like to alter the backlight unit or to break the LED lamps 31, thereby degrading color uniformity and color mixture rate. To solve the problem, the intermediate light-guiding panel 32 and the reflector sheets 33 may be removed. However, if they are removed, the light coming from the upper part of the LED lamps 31 directly propagate to the diffuser sheet to fail in being mixed together, thereby degrading the light uniformity.